Optical pick-up device for a multi-layer recording medium with a photodetector arrangement for focusing and tracking control

ABSTRACT

An optical pick-up device invention is adapted according to the present invention to reproduce an information signal recorded on a multi-layer optical disc having a plurality of information signal storage layers on which information signals are stored. The optical pick-up device includes a light source for emitting a light beam, an optical means for dividing the light beam emitted from the light source into three separate beams composed of a main beam and first and second side beams and radiating these beams onto the multi-layer optical disc, a main beam-receiving member for receiving the main beam reflected from the multi-layer optical disc, first and second side beam-receiving members for receiving the first and second side beams reflected from the multi-layer optical disc, respectively, and a means for outputting a detection signal indicative of a quantity of light in response to the light quantity received by the main beam-receiving member, the first side beam-receiving member and the second side beam-receiving member. The first side beam-receiving member and the second side beam-receiving member are spaced a given distance apart from the main beam-receiving member so that no interference with the main beam reflected from an unfocused information signal storage layer of the multi-layer optical disc is caused, so that the light beam reflected from the unfocused information signal storage layer of the multi-layer optical disc does not have any influence on function of the device.

This application is a 371 of PCT/JP95/02770 filed Dec. 28, 1995.

TECHNICAL FIELD

The present invention relates to an optical pickup device for use in arecording and/or reproducing apparatus capable of recording aninformation signal on a so-called multi-layer optical disc having aplurality of information layers and/or reproducing the informationsignal from respective information storage layers of the multi-layeroptical disc.

BACKGROUND ART

Optical pickup devices used in conventional recording and/or reproducingapparatuses in which an optical disc is used as a recording medium,includes a semiconductor laser as a light source for emitting a lightbeam, a collimator lens which is disposed between the semiconductorlaser and the optical disc and whose optical axis is aligned withoptical axes of the semiconductor laser and the optical disc, adiffraction grating, a polarizing beam splitter and an objective lens.The optical pickup device further includes a photodetector for receivinga light beam from the optical disc, and a focusing lens which isdisposed between the photodetector and the polarizing beam splitter andwhose optical axis is aligned with optical axes of the photodetector andthe polarizing beam splitter.

In such conventional optical pickup devices, the light beam is emittedfrom the semiconductor laser and transmitted through the polarizing beamsplitter and an objective lens so as to be radiated and focused onto aninformation signal storage layer of the optical disc. The light beamreflected from the optical disc is received by the photodetector.

The optical disc used as a recording medium for a recording and/orreproducing apparatus in which the afore-mentioned optical pick-up isincorporated and which is adapted to record an information signalthereon and/or reproduce the signal therefrom, generally includes asubstrate made of a transparent synthetic resin material such aspolycarbonate or polymethyl-methacrylate, an information signal layerformed on a main surface of the substrate, and a protecting layer formedover the information signal layer to protect the layer against chemicalerosion or mechanical damage.

Such an optical disc has been produced by using various shaping methodssuch as a thermal pressure-molding method or a thermal casting method.When subjected to the shaping method, the optical disc is likely tosuffer from undesired warpage or corrugation due to a thermal distortiongenerated therein. Consequently, a dislocation of the information signalstorage layer occurs in the direction perpendicular to the main surfaceof the disc. Further, the circularity of tracks formed on theinformation signal layer is adversely affected. When the optical disc isrotated to reproduce the information signal stored on the informationsignal layer, the surface of the information signal layer and the tracksthereon suffer from undesirable oscillation and deflection.

Accordingly, in order to achieve an accurate read-out or reproduction ofthe information signal stored on a signal recording region of theoptical disc without being adversely affected by the oscillation of theoptical disc surface and the deflection of the tracks, the opticalpickup device is provided with a focusing control function forconducting an adequate focusing adjustment of the objective lens inresponse to a degree of the eventual oscillation of the surface of theoptical disc, and a tracking control function for performing an adequatetracking adjustment of the objective lens so as to follow theoscillation of a signal track formed on the signal recording region ofthe optical disc.

Meanwhile, there has been a demand for higher density informationstorage, so that proposals have been made to provide a multi-layeroptical disc having a multiple of information signal layers arranged inan overlapped relation to each other. The multi-layer optical discconventionally known is of a two-layer type having two overlappedinformation signal layers.

The two-layer type optical disc 5 includes, as shown in FIG. 1, a discsubstrate 5C made of a transparent synthetic resin material such aspolycarbonate (PC) or polymethyl-methacrylate (PMMA), a firstinformation signal storage layer 5A formed on a main surface of the discsubstrate 5C, a spacer layer 5D made of a transparent resin material anddisposed over the first information signal storage layer, a secondinformation signal storage layer 5B disposed over the first informationsignal storage layer 5A through the spacer layer 5D, and a protectinglayer 5E formed over the second information signal storage layer 5B toprotect the second information signal storage layer against mechanicaldamage and chemical erosion.

In such a two-layer type optical disc, the laser beam emitted from thesemiconductor laser is transmitted through the first information signalstorage layer 5A and radiated to the second information signal storagelayer 5B when the optical pickup device performs a read-out of theinformation signal from the second information signal storage layer 5Bdisposed over the first information signal storage layer 5A.

In the optical pickup device used here, the tracking control is carriedout by using a 3-spot method in which three light beams are radiated tothe optical disc to obtain a tracking control signal. When theinformation signal stored on the individual information signal storagelayers of the multi-layer optical disc is read by means of the opticalpickup device which employs the above-mentioned 3-spot method to conducta tracking control, the following problems are caused. As shown in FIG.2, a photo-detector used in such an optical pick-up device in which atracking control is performed by using the 3-spot method, includes amain beam detector 33A for detecting a main beam which is a centrallyradiated beam of three separated light beams, and first and secondside-beam detectors 33B and 33C disposed on opposite sides of the mainbeam detector 33A, arranged so as to sandwich the main beam detector 33Atherebetween, for detecting side beams radiated on opposite sides of themain beam.

When the information signal stored on the first information signalstorage layer 5A of the two-layer type optical disc 5 is read out, themain beam 34A and the first and second side beams 34B and 34C areradiated onto the first information signal storage layer 5A. In thecourse of the reading-out of the information signal stored on the firstinformation signal storage layer 5A, these light beams are transmittedthrough the first information signal storage layer 5A and also radiatedonto the second information signal storage layer 5B whose informationsignal is not intended to be read out.

In consequence, the photo-detector of the optical pick-up devicereceives not only the main beam 34A reflected from the focused firstinformation signal storage layer 5A but also a blooming light component35A of the main beam, namely a stray light component, reflected from theunfocused information signal storage layer SB. That is, in the opticalpick-up device, the main beam 34A and the stray light component 35Ahaving an expanded outer perimeter are radiated together onto the mainbeam detector 33A in a concentric relation to each other.

Accordingly, in such an optical pick-up device, a part of the straylight component 35A of the main beam radiated on the main beam detector33A is overlappingly radiated over the side beam detectors 33B and 33C.Thus, when the part of the stray light component 35A of the main beam isradiated over the side-beam detectors 33B and 33C and overlapped withthe respective side beams, there occurs direct-current (DC) fluctuationin a tracking error signal, which occasionally makes it impossible tocarry out an accurate tracking control.

Consequently, in the optical pick-up device, such an advantage of the3-spot method that the tracking error signal is not adversely affectedby a skew of the disc and a deflection of a view field of the objectivelens and therefore does not suffer from D.C. fluctuation, cannot bemaintained. Further, when the radiation of a read light beam focusedthrough the objective lens is switched between the information signalstorage layers of the two-layer type optical disc, a form of the straylight component 35A of the main beam radiated on the main beam detector33A is changed.

Accordingly, a ratio of a light quantity of the stray light component35A of the main beam radiated over the respective side beam detectors33B and 33C to those of the side beams is also changed, so that thetracking error signal suffers from D.C. offset, thereby causing such aproblem that it is no longer possible to perform the accurate trackingcontrol.

Meanwhile, an intensity of the stray light component of each side beamis considerably that of the stray light component of the main beam. Inconsequence, the influence of the stray light components of the sidebeams on the main beam detector is negligible and therefore causes nosignificant problem.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an optical pick-updevice capable of accurately reproducing an information signal recordedon a plurality of information signal storage layers of a multi-layeroptical disc.

It is another object of the present invention to provide an opticalpick-up device capable of conducting a tracking control with a highaccuracy when an information signal stored on a multi-layer optical discis read out, whereby an accurate reproduction of the information signalcan be achieved.

In order to accomplish the afore-mentioned objects, in accordance withthe present invention, there is provided an optical pick-up device whichincludes a light source for emitting a light beam to be radiated towardthe multi-layer optical disc having a plurality of information signalstorage layers which are arranged in a overlapped manner and on each ofwhich a desired information signal is recorded, an optical means fordividing the light beam emitted from the light source into threeseparate beams composed of a main beam and first and second side beamsand radiating these beams onto the multi-layer optical disc, a mainbeam-receiving member for receiving the main beam reflected from themulti-layer optical disc, a first side beam-receiving member forreceiving the first side beam reflected from the multi-layer opticaldisc, a second side beam-receiving member for receiving the second sidebeam reflected from the multi-layer optical disc, and a means foroutputting a detection signal indicative of a quantity of light inresponse to the light received by the main beam-receiving member, thefirst side beam-receiving member and the second side beam-receivingmember. The first side beam-receiving member and the second sidebeam-receiving member are disposed a given distance apart from the mainbeam-receiving member and located at positions such that no interferencewith the main beam reflected from an unfocused information signalstorage layer of the multi-layer optical disc is caused.

The optical pick-up device used here is provided with a diffractiongrating for dividing the light beam emitted from the light source intothe three separate beams composed of the main beam and the first andsecond side beams. The diffraction grating divides the light beamemitted from the light source into the three separate beams which arespaced apart from each other by such a given distance that the main beamreflected from an unfocused information signal storage layer causes nointerference with said first side beam-receiving member and said secondside beam-receiving member. In addition, the optical pick-up device isprovided with an optical element for generating an astigmatism.

In the optical pick-up device, a focusing error is detected based on adetection result from the main beam-receiving member.

Further, a tracking error is detected based on a detection results fromthe first side beam-receiving member and the second side-beam-receivingmember.

In another aspect of the present invention, there is provided an opticalpick-up device which includes a light source for emitting a light beamto be radiated toward the multi-layer optical disc having a plurality ofinformation signal storage layers which are arranged in a overlappedmanner and on each of which a desired information signal is recorded, anoptical means including a diffraction grating for dividing the lightbeam emitted from the light source into three separate beams composed ofa main beam and first and second side beams, and an objective lens forradiating the three separate beams divided by the diffraction gratingonto the multi-layer optical disc in such a manner that the respectivebeams are focused on the information signal storage layer of themulti-layer optical disc, a main beam-receiving member for receiving themain beam reflected from the multi-layer optical disc, a first sidebeam-receiving member for receiving the first side beam reflected fromthe multi-layer optical disc, a second side beam-receiving member forreceiving the second side beam reflected from the multi-layer opticaldisc and a means for outputting a detection signal indicative of aquantity of light in response to the light received by the mainbeam-receiving member, the first side beam-receiving member and thesecond side beam-receiving member. The first side beam-receiving memberand the second side beam-receiving member are disposed a given distanceapart from the main beam detector and located at such positions that nointerference with the main beam reflected from an unfocused informationsignal storage layer of the multi-layer optical disc is caused.

In a further aspect of the present invention, there is provided anoptical pick-up device which includes a light source for emitting alight beam to be radiated toward said multi-layer optical disc having aplurality of information signal storage layers which are arranged in aoverlapped manner and on each of which a desired information signal isrecorded, an optical means including a diffraction grating for dividingthe light beam emitted from the light source into three separate beamscomposed of a main beam and first and second side beams, an objectivelens for radiating the three separate beams divided by the diffractiongrating onto the multi-layer optical disc in such a manner that therespective beams are focused on the information signal storage layer ofthe multi-layer optical disc, and a polarizing beam splitter on whichthe light beam reflected from the multi-layer optical disc is incident,first and second photo-detectors each having a main beam-receivingmember for receiving the main beam reflected from the multi-layeroptical disc and transmitted through or reflected on the polarizing beamsplitter, a first side beam-receiving member for receiving the firstside beam reflected from the multi-layer optical disc and transmittedthrough or reflected on the polarizing beam splitter, a second sidebeam-receiving member for receiving the second side beam reflected fromthe multi-layer optical disc and transmitted through or reflected on thepolarizing beam splitter, and a means for outputting a detection signalindicative of a quantity of light in response to the light received bythe main beam-receiving member, the first side beam-receiving member andthe second side beam-receiving member provided on each of said first andsecond photo-detectors. The first side beam-receiving member and thesecond side beam-receiving member provided on each of said first andsecond photo-detectors are disposed a given distance apart from the mainbeam detector and located at such positions that no interference withthe main beam reflected from an unfocused information signal storagelayer of the multi-layer optical disc is caused.

Here, the main beam-receiving member provided in each of the first andsecond photo-detectors is divided into three strip-like light-receivingsub-members.

Other objects and advantages of the present invention will become moreapparent by referring to the following detailed description of thepreferred embodiment of the present invention when read in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing an optical disc having twoinformation signal storage layers on which an information signal to bereproduced by an optical pick-up device according to the presentinvention is recorded;

FIG. 2 is a plan view showing a photo-detector to explain problems posedon a conventional optical pick-up device;

FIG. 3 is a side view schematically showing an optical pick-up deviceaccording to a first embodiment of the present invention;

FIG. 4 is a plan view showing a photo-detector provided in the opticalpick-up device according to the first embodiment of the presentinvention;

FIGS. 5A and 5B are plan views showing the condition of a stray lightattributed to a main beam reflected from respective information signalstorage layers and incident on a main beam detector of thephoto-detector;

FIG. 6 is a plan view showing the condition of a light beam and a straylight incident on a main beam detector and side beam detectors of thephoto detector;

FIG. 7 is a plan view showing a photo-detector provided in an opticalpick-up device according to a second embodiment of the presentinvention;

FIG. 8 is a side view schematically showing an optical pick-up deviceaccording to a third embodiment of the present invention;

FIG. 9 is a plan view showing a photo-detector provided in the opticalpick-up device according to the third embodiment of the presentinvention;

FIG. 10 is a schematic view serving for explaining equations forcalculating a spot diameter of a stray light attributed to a main beamand radiated onto a main beam detector provided in an optical pick-updevice for an optical disc; and

FIG. 11 is a plan view showing a photo-detector provided in an opticalpick-up device according to a fourth embodiment of the presentinvention.

PREFERRED EMBODIMENT OF THE INVENTION

In the foregoing, an optical pick-up device according to preferredembodiments of the present invention is described. The optical pick-updevice is adapted to be incorporated in a recording and/or reproducingapparatus in which a multi-layer optical disc having two informationsignal storage layers is used as a recording medium.

With the optical pick-up device according to the present invention, thetracking control is performed by detecting a tracking error by using a3-spot method and the focusing control is performed by detecting afocusing error by using an astigmatism method.

In addition, the two-layer optical disc 5 used in the recording and/orreproducing apparatus to which the optical pick-up device according tothe present invention is applied, includes, as shown in FIG. 1, a discsubstrate 5C made of a transparent synthetic material such aspolycarbonate (PC) or polymethyl-methacrylate (PMMC), a firstinformation signal storage layer 5A formed over a primary surface of thedisc substrate 5C, a spacer layer 5D made of a transparent resinmaterial and formed over the first information signal storage layer 5A,a second information signal storage layer 5B formed over the firstinformation signal storage layer 5A through the spacer layer 5D, and aprotecting layer 5E formed over the second information signal storagelayer 5B.

In such an optical disc, when the information signal stored on thesecond information signal storage layer 5B is read out by means of theoptical pick-up device according to the present invention, a light beamemitted from a semiconductor laser is transmitted through the firstinformation signal storage layer 5A and radiated onto the secondinformation signal storage layer 5B.

The optical pick-up device 1 according to a first embodiment of thepresent invention includes, as shown in FIG. 3, a semiconductor laser 6serving as a light source for emitting a light beam, a collimator lens 7disposed between the semiconductor laser 6 and the two-layer opticaldisc 5 such that optical axes thereof are aligned with each other, adiffraction grating 8, a polarizing beam splitter (PBS) 9, 1/4wave-length plate 10 and an objective lens 11. Furthermore, the opticalpick-up device 1 has a photo-detector 14 for receiving a light beamreflected from the optical disc 5, a focusing lens 12 provided betweenthe photo-detector 14 and the polarizing beam splitter 9 such thatoptical axes thereof aligned with each other, and a cylindrical lens 13.

As shown in FIG. 4, the photo-detector 14 is composed of a main beamdetector 26A for receiving a centrally-incident main beam of threeseparate light beams, and first and second side beam detectors 26B and26C for receiving the side beams radiated on opposite sides of the mainbeam. Accordingly, the first and second side beam detectors 26B and 26Cof the photo-detector are arranged on opposite sides of the main beamsuch that the main beam detector 26A is interposed therebetween.

The main beam detector 26A which is a constituent of the photo-detector14 is composed of four segments, whereby a light receiving surfacethereof is divided into first to fourth light receiving regions 41A to41D along separate lines L₁ and l₂ as shown in FIG. 4, which areperpendicular to each other and crossed at a center (optical axis) ofthe main beam 27A of the reflected light incident on the light receivingsurface, namely a central point P₁ of the light receiving surface, asshown in FIG. 4.

As shown in FIGS. 4, 5A and 5B, the first side beam detector 26B and thesecond side beam detector 26C are spaced apart from the main detector26A by such a distance that the side beam detectors are not radiatedwith a stray light 28A attributed to the main beam of the reflectedlight coming from the information signal storage layer, on which thelight beam radiated through the objective lens 11 is not focused andwhose information signal is not intended to be read out. That is, thefirst and second side beam detectors 26B and 26C are so arranged suchthat reference center lines Q₁ and Q₂ thereof and the separate line L₁of the main beam detector 26A, which extend perpendicularly to thedirection of a row of the detectors 26A to 26C as shown in FIGS. 4, 5Aand 5B, are spaced apart from each other by a distance A₁ which issufficient to prevent the first and second side beam detectors frombeing radiated with the stray light 28A attributed to the main beam ofthe light reflected from the unfocused information signal storage layerwhose information signal is not intended to be read out.

An optical path of the light beam emitted from the semiconductor laser 6and the occurence of the stray light 28A of the main beam incident onthe main beam detector 26A during read-out and reproduction of theinformation signal from the first information signal storage layer ofthe two-layer optical disc 1, are explained by using the afore-mentionedarrangement of the optical pick-up device 1.

First, the light beam emitted from the semiconductor laser 6 is incidenton the collimator lens 7. The collimator lens 7 permits the incidentlight beam to pass through converting it from a divergent light to aparallel one. The light beam passing through the collimator lens 7 isthen incident on the diffraction grating 8.

The diffraction grating 8 divides the incident light beam into threeseparate light beams including the main beam 27A, the first side beam27B and the second side beam 27C, which in turn are incident on thepolarizing beam splitter 9. Thus, the polarizing beam splitter 9 permitsthe respective light beams split from the light beam emitted from thesemiconductor laser 6 to pass toward the two-layer optical disc 5. Thepolarizing beam splitter 9 reflects the light reflected from thetwo-layer optical disc 5, as described hereinafter.

The respective light beams passing through the polarizing beam splitter9 are incident on the 1/4 wave-length plate 10. When passing through the1/4 wave-length plate 10, each light beam is converted from a linearlypolarized light beam to a circularly polarized light beam. The lightbeams passing through the 1/4 wave-length plate 10 are incident on theobjective lens 11.

The objective lens 11 condenses the incident light beams and radiatesthem onto the first information signal storage layer 5A of the two-layeroptical disc 5. At this time, the respective light beams radiated on thefirst information signal storage layer 5A of the two-layer optical disc5 are not only focused on the first information signal storage layer 5Aof the two-layer optical disc 5 but also passed therethrough andradiated onto the information signal storage layer 5B.

The light beams radiated onto the first information signal storage layer5A of the two-layer optical disc 5 are reflected on the firstinformation signal storage layer 5A and also reflected by the secondinformation signal storage layer 5B. That is, the light beams radiatedonto the first information signal storage layer 5A of the two-layeroptical disc 5 are reflected not only by the first information signalstorage layer 5A on which the incident light passing through theobjective lens 11 is focused, but also on the second information signalstorage layer 5B on which the incident light passing through theobjective lens 11 is not focused. Each of the light beams reflected fromthe second information signal storage layer 5B constitute a stray lightfor each of the light beams reflected from the first information signalstorage layer 5A.

The reflected light beams and the stray light beams are incident on theobjective lens 11 and pass therethrough so as to be incident on the 1/4wave-length plate 10. The 1/4 wave-length plate 10 converts each of theincident reflected light beams and stray light beams form a linearlypolarized light to a circularly polarized light. The reflected lightbeams and the stray light beams pass through the 1/4 wave-length plate10 and are incident on the polarizing beam splitter 9. The polarizingbeam splitter 9 reflects the reflected light beams and the stray lightbeams . The light beams and the stray light beams reflected by thepolarizing beams splitter 9 are then incident on the focusing lens 12and condensed when passed therethrough.

The reflected light beams and the stray light beams passing through thefocusing lens 12 are incident on the cylindrical lens 13. The reflectedlight beams and the stray light beams incident on the cylindrical lens13 are further condensed and then radiated onto the respective detectorsof the photo-detector 14. At this time, there occurs an astigmatismbetween the reflected light beams and the stray light beams passingthrough the cylindrical lens 13.

The main beam detector 26A of the photo-detector 14 generates a focusingcontrol at the objective lens 11 by determining a quantity of the lightreceived by the first to fourth light receiving regions 41A to 41Dthereof. A focusing error signal FE, which is indicative of a defocusingdegree of the light radiated onto a surface of the first informationsignal storage layer 5A of the two-layer optical disc 5, can becalculated from the following equation:

    FE=(E1+E3)-(E2+E4)

where E1, E2, E3 and E4 represent detection outputs generated from thelight-receiving regions 41A, 41B, 41C and 41D of the main beam detector26A, respectively, in response to the amounts of the light beamsreceived thereby.

The focusing control of the objective lens 11 can be performed based onthe thus-obtained focusing error signal FE.

On the other hand, the first and second side beams 27B and 27C areincident on the first and second side beam detectors 26B and 26C of thephoto-detector 14, respectively. A tracking error signal TE, which isindicative of dislocation of the main beam relative to recording tracksformed on the first information signal storage layer 5A of the two-layeroptical disc 5, can be calculated from the following equation:

    TE=E5E6

where E5 and E6 represent detection outputs generated from the first andsecond side beam detectors 26B and 26C in response to the quantities ofthe light beams-received thereby.

The tracking control of the objective lens 11 can be performed based onthe thus-obtained tracking error signal TE.

Meanwhile, the main beam 27A of the light reflected from the firstinformation signal storage layer 5A is incident on the main beamdetector 26A, as shown in FIGS. 5A and 5B, while the first and secondside beams 27B and 27C of the light reflected from the first informationsignal storage layer 5A are incident on the side beam detectors 26B and26C, respectively. Furthermore, the stray light beam 28A attributed tothe main beam of the light reflected from the second information signalstorage layer 5B is incident on the main beam detector 26A, while straylight beams 28B and 28C attributed to the first and second side beams,respectively, reflected from the second information signal storage layer5B, are incident on the first and second side beam detectors 26B and26C, respectively, as shown in FIGS. 5A and 5B.

As described above, the first and second side beam detectors 26A and 26Bare spaced apart from the main beam detector 26A by a given distancesufficient to avoid influence of the stray light beam 28A attributed tothe main beam, so that the stray light beam 28A incident on the mainbeam detector 26A is prevented from being radiated over the first andsecond side beam detectors 26B and 26C as shown in FIGS. 5A and 5B.Also, the stray light beams 28B and 28C attributed to the first andsecond side beams cannot be radiated over the main beam detector 26Athough not shown in the figures.

In addition, even if the stray light beams 28B and 28C attributed to thefirst and second side beams from the second information signal storagelayer 5B is radiated over the main beam detector 26A, no significantinfluence is caused on the main beam detector 26A because these straylight beams have a considerably lower light intensity as compared tothat of the stray light beam 28A attributed to the main beam.

In the optical pick-up device 1 according to the present invention, thestray light beam 28A attributed to the main beam forms an elliptic spot,as shown in FIGS. 5A and 5B, due to an astigmatism caused by thecylindrical lens 13. Further, major and minor axes of the elliptic spotof the stray light beam 28A attributed to the main beam appear in areverse relation depending upon whether it reflects from the firstinformation signal storage layer 5A or from the second informationsignal storage layer 5B.

That is, in the optical pick-up device according to the presentinvention, in the event that the information signal to be read out andreproduced is stored on the second information signal storage layer 5Bof the two-layer optical disc 5, which is located further from theobjective lens 11, the stray light beam 28A attributed to the main beamin the light reflected from the first information signal storage layer5A forms an elliptic spot having a major axis inclined at an angle of 45degrees rightwardly as shown in FIG. 5A.

Conversely, in the event that the information signal to be read out andreproduced is stored on the first information signal storage layer 5A ofthe two-layer optical disc 5, which is located closer to the objectivelens 11, the stray light beam 28A attributed to the main beam in thelight reflected from the second information signal storage layer 5Bforms an elliptic spot having a major axis inclined at an angle of 40degrees leftwardly as shown in FIG. 5B.

In the optical pick-up device according to the present invention, whenthe two-layer optical disc 5 is caused to incline in a radial directionor when the objective lens 11 undergoes a change in visual field , thestray light 28A attributed to the main beam or distribution of lightintensity thereof is shifted on the main beam detector 26A in thedirection of an arrow M as indicated in FIGS. 5A, 5B and 6. However, thestray light 28A attributed to the main beam and shifted on the main beamdetector 26A is not radiated over the first and second side beamdetectors 26B and 26C, so that no D.C. offset of the tracking errorsignal occurs.

As described above, in the optical pick-up device according to thisembodiment of the present invention, the first and second side beamdetectors 26B and 26C are disposed outside of a spot diameter of thestray light 28A, whereby the first and second side beam detectors 26Band 26C are exposed to no overlapped radiation of the stray light 28Aattributed to the main beam. Accordingly, in the optical pick-up deviceaccording to the present invention, since the D.C. offset of thetracking error signal is effectively prevented, accurate trackingcontrol of the light beam radiated over the two-layer optical disc 5 canbe carried out.

Further, in the optical pick-up device according to the presentinvention, even if the focal point of the light beam radiated onto thetwo-layer optical disc is switched from one information signal storagelayer to another layer due to a change in information signal to be readout and reproduced, no D.C. offset of the tracking error signal occurs.Consequently, the tracking control of the light beam radiated over thetwo-layer optical disc 5 can be accurately carried out even in such acase.

Furthermore, in the optical pick-up device according to the presentinvention, the focusing control is performed by using an astigmatismmethod, which enables not only miniaturization of the apparatus as awhole but also improvement in sensitivity for detection of the focusingerror signal.

In the afore-mentioned arrangement of the optical pick-up deviceaccording to this embodiment of the present invention, the polarizingbeam splitter 9 is employed. However, the polarizing beam splitter 9 isreplaced with a normal beam splitter, whereby the 1/4 wave-length plate10 can be omitted. That is, by using the normal beam splitter in placeof the polarizing beam splitter 9, it is not necessary to use the 1/4wave-length plate 10. This permits not only a simplified construction ofthe apparatus as a whole, but also reduction in manufacturing costthereof.

In the afore-mentioned optical pick-up device according to the firstembodiment of the present invention, the light beam emitted from thesemiconductor laser 6 is introduced to the diffraction grating 8 so asto be divided into the main beam 27A and the first and second side beams27B and 27C, which are spaced by a given distance apart from each other.The main beam detector 26A and the first and second side beam detectors26B and 26C are so arranged as to be spaced by a constant distance Alfrom each other, as shown in FIG. 4. However, in order to prevent thefirst and second side beam detectors from being adversely affected bythe stray light attributed to the main beam, there can be also providedan optical pick-up device 2 according to a second embodiment of thepresent invention, as explained below.

The optical pick-up device according to the second embodiment of thepresent invention has essential parts common to those of the opticalpick-up device 1 according to the first embodiment of the presentinvention. Therefore, like parts are identified by like referencenumerals and detailed explanation thereof are omitted here.

In the optical pick-up device according to the second embodiment of thepresent invention, as shown in FIG. 7, a photo-detector 15 is composedof a main beam detector 36A and first and second side beam detectors 36Aand 36B disposed on opposite sides of the main beam detector 36A suchthat the main beam detector 36A is interposed therebetween.

The main beam detector 36A of the photo-detector 15 is of four-segmentconstruction and has first to fourth light-receiving regions 47A to 47Ddivided along separate lines L₂ and l₁ which extend perpendicularly toeach other and are crossed at a center (optical axis) of the main beam27A of the light reflected from the two-layer optical disc 5 andradiated onto a light-receiving surface thereof, namely a central pointP.

The first side beam detector 36B is partially cut out as indicated by abroken line in FIG. 7 on a side where the stray light 28A is radiated inan overlapped relation to the side beams, namely a side facing the mainbeam detector 36A. The stray light 28A in the form of a beam spot isattributed to the main beam of the light beams reflected from theunfocused information signal storage layer whose information signals arenot intended to be read out, and incident on the main beam detector 36A.The second side beam detector 36C is also partially cut out as indicatedby a broken line in FIG. 7, on a side where the stray light 28A incidenton the main beam detector 36A is radiated in an overlapped relation tothe side beams, namely a side facing the main beam detector 36A.

That is, both the first and second side beam detectors 36B and 36C arepartially cut out at portions located on inner sides thereof relative toreference center lines Q₃ and Q₄ extending perpendicularly to thedirection of the row of the respective detectors 36A to 36C, as shown inFIG. 7.

The optical pick-up device 2 having the afore-mentioned arrangement canbe also prevented from being adversely affected by the stray light 28Aattributed to the main beam of the light reflected from the informationsignal storage layer on which the light beams are not focused and whoseinformation signals are not intended to be read out.

Next, an optical pick-up device 3 according to a third embodiment of thepresent invention is explained below. In the optical pick-up device, atracking control is carried out by using the 3-spot method while afocusing control is carried out by using a differential concentriccircles method.

As shown in FIG. 8, similarly to those of the preceding embodiments, theoptical pick-up device according to this embodiment also includes asemiconductor laser 16 serving as a light source for emitting a lightbeam, a collimator lens 17 disposed between the semiconductor laser 16and the two-layer optical disc 5 such that optical axes thereof arealigned with each other, a diffraction grating 18, a beam splitter (BS)19, a 1/4 wave-length plate 20 and an objective lens 21. Further, theoptical pick-up device 3 is provided with a photo-detector 24 forreceiving a light beam reflected from the two-layer optical disc 5, afocusing lens 22 disposed between the photo-detector 24 and the beamsplitter (BS) 19 such that optical axes thereof are aligned with eachother, and a polarizing beam splitter (PBS) 23.

The photo-detector 24 is composed of a first photo-detector 24A and asecond photo-detector 24B, as shown in FIG. 8. As shown in FIG. 9, eachof the first and second photo-detectors 24A and 24B has a main beamdetector 30A and first and second side beam detectors 30B and 30Cdisposed on opposite sides of the main beam detector 30A so as tointerpose the main beam detector 30A therebetween.

The main beam detector 30A is composed of three separate strip-likeplates which are disposed in parallel with each other and define first,second and third light-receiving regions 44A, 44B and 44C, respectively.The first and second side beam detectors 30B and 30C are in the form ofa single plate and therefore have one light-receiving region only. Thefirst and second side beam detectors 30B and 30C are disposed outside ofa spot diameter of a stray light 32A attributed to the main beamreflected from the information signal layer on which the light beam arenot focused and whose information signals are not intended to be readout, and incident on the main beam detector 30A.

That is, as shown in FIG. 9, the first and second side beam detectors30B and 30C are spaced a given distance apart from the main beamdetector 30Aby a distance A₂ so as not to receive the stray light 32Aattributed to the main beam reflected from the information signalstorage layer from which no information signal is intended to be readout, is formed between each of the reference center lines Q₃ and Q₄ Ofthe first and second side beam detectors and the center line L₃ Of themain beam detector 30A.

A spot diameter b of the stray light 32A attributed to the main beam,which is reflected from the information signal storage layer from whichno information signal is intended to be read out and radiated onto themain beam detector 30A, is calculated from the following equation (1),in the event that the afore-mentioned astigmatism method is not used forconducting a focusing control as shown in FIG. 10In the equation (1):

d: distance between two information signal storage layers of thetwo-layer optical disc 5 (a thickness of the spacer layer 5D);

f₁ : focal length of the objective lens 21;

f₂ : focal length of the focusing lens 22;

Δf: distance between a focal point 29A of the stray light 32A attributedto the main beam and a focal plane 29C of the focusing lens 22;

x: distance between a light-receiving surface 29B of the main beamdetector 30A and the focal plane 29C of the focusing lens 22;

b: spot diameter of the stray light 32A attributed to the main beam andradiated on the light-receiving surface 29B of the main beam detector30A; and

NA: numerical aperture of the objective lens 21.

Here, the value d is obtained by using the focal plane as a referenceplane while the values Δf and x are determined by using the focal plane29C of the focusing lens 22 as a reference plane. Incidentally, adimension taken in the proceeding direction of the reflected light isrepresented in a positive (+) value.

That is, the spot diameter b of the stray light 32A is given by thefollowing equation (1): ##EQU1## where h represents a lateralmagnification of the reflected light detection system and satisfies therelationships of Δf=2n² d and η=f₂ /f₁. Therefore, a spot radius b/2 ofthe stray light is expressed by the following equation (2): ##EQU2##

Further, if an actual value of d is d' and a refractive index of thedisc substrate is n, the following relationship is established:

    d'=|nd|(d'>0)

Accordingly, when the information signal stored on the first informationsignal storage layer 5A of the two-layer optical disc 5 is intended tobe read out and reproduced, a minimum value of the distance A₂ betweeneach of the reference center lines Q₃ and Q₄ Of the first and secondside beam detectors 30B and 30C and the center line L₃ Of the main beamdetector 30A is given by the following equation (3): ##EQU3##

On the other hand, if the information signal stored on the secondinformation signal storage layer of the two-layer optical disc 5 isintended to be read out and reproduced, the minimum value is given bythe following equation (4): ##EQU4##

The minimum value of the distance A₂ between each of the referencecenterlines Q₃ and Q₄ Of the first and second side beam detectors 30Band 30C and the center line L₃ of the main beam detector 30A isdetermined as a larger one of the values obtained from the equations (3)and (4).

In the afore-mentioned arrangement of the optical pick-up device 3according to the third embodiment of the present invention, an opticalpath of the light beam emitted from the semiconductor laser 16 and theoccurrence of the stray light 32A of the main beam radiated on the mainbeam detector 30A will be explained with reference to FIGS. 8 and 9 forthe case in which the information signal stored on the first informationsignal storage layer 5A of the two-layer optical disc 5 is intended tobe read out.

The light beam emitted from the semiconductor laser 16 is first incidenton the collimator lens 17. The collimator lens 17 transmits the incidentlaser beam and converts it from a divergent light to a parallel light.The light beam passes through the collimator lens 17 and is thenincident on the diffraction grating 18. When transmitted through thediffraction grating 18, the light beam is divided into three separatebeams composed of a main beam 31A and two side beams 31B and 31C. Thethree separate beams are incident on the beam splitter 19. Therespective beams are allowed to pass through the beam splitter 19. Thelight beams passing through the beam splitter 19 are then incident onthe objective lens 21. The objective lens 21 condenses the incidentlight beams and radiates the condensed light beams onto the firstinformation signal storage layer 5A of the two-layer optical disc 5. Atthis time, the respective light beams directed to the first informationsignal storage layer 5A of the two-layer optical disc 5 are is radiatednot only onto the first information signal storage layer 5A but alsotransmitted through the first information signal storage layer 5A andalso radiated onto the second information signal storage layer 5B.

The light beams radiated onto the first information signal storage layer5A are reflected therefrom and the light beams radiated onto the secondinformation signal storage layer 5B are also reflected therefrom. Thatis, the reflected light attributed to the light focused by the objectivelens 21 on the first information signal storage layer 5A are mixed withthe stray light beams reflected from the second information signalstorage layer 5B on which the focusing of the objective lens 21 is notperformed.

Thus, the mixed light beams reflected from the first information signalstorage layer 5A and the second information signal storage layer 5B areincident on the objective lens 21. The objective lens 21 transmits theincident light beams reflected from the first information signal storagelayer 5A and the second information signal storage layer 5B. The lightbeams are directed to the beam splitter 19 so as to be incident thereon.

When transmitted through the beam splitter, the respective incidentlight beams reflected from the first information signal storage layer 5Aand the second information signal storage layer 5B are reflected thereonand directed towards the focusing lens 22 so as to be incident thereon.The incident light beams are converged by the focusing lens 22 whentransmitted therethrough.

The light beams reflected from the first information signal storagelayer 5A and the second information signal storage layer 5B are incidenton the polarizing beam splitter 23. The polarizing beam splitter 23divides each of the light beams reflected from the first informationsignal storage layer 5A and the second information signal storage layer5B into two separate beams by reflecting a part of each light beam on aseparating surface S thereof and transmitting the remainder of the lightbeam therethrough. The divided beams are respectively radiated onto thefirst photo-detector 24A and the second photo-detector 24B of thephoto-detector 24.

The main beam detector 30A provided on each of the first photo-detector24A and the second photo-detector 24B produces a focusing error signalin response to a detection output indicative of a light quantity of themain beam 31A incident on first to third light-receiving regions 44A to44C thereof. The focusing control for the objective lens 21 is performedbased on the thus-obtained focusing error signal.

In the foregoing, the condition for obtaining the focusing error signalin the optical pick-up device 3 according to the present embodiment isspecifically explained below.

In the optical pick-up device 3, the focusing error signal can beobtained based on the detection outputs generated from the main beamdetector 30A of the first photo-detector 24A and the main beam detector30B of the second photo-detector 24B.

In a case where the detection outputs based on light quantities of thelight beams received by the first, second and third light-receivingregions 44A, 44B and 44C of the main beam detector 30A of the firstphoto-detector 24A are indicated by Ell, E12 and E13, respectively, afirst focusing error signal EF1 can be obtained from the detectionoutput of the main beam detector 30A according to the following equation(5):

    EF1=Ell-(E12+E13)                                          (5)

Next, in a case where the detection outputs based on light quantities ofthe light beams received by the first, second and third light-receivingregions 44A, 44B and 44C of the main beam detector 30B of the secondphoto-detector 24B are indicated by e11, e12 and e13, respectively, asecond focusing error signal EF2 can be obtained from the detectionoutput of the main beam detector 30B according to the following equation(6):

    EF2=e11-(e12+e13)                                          (6)

Further, a focusing error signal EF of the optical pick-up device 3 canbe obtained by subtracting the second focusing error signal EF2 from thefirst focusing error signal EF1 according to the following equation (7):

    EF=EF1-EF2                                                 (7)

On the other hand, a tracking error signal of the optical pick-up device3 can be obtained from the detection outputs of the first side beamdetector 30B and the second side beam detector 30C provided on each ofthe first and second photo-detectors 24A and 24B.

Here, in a case where detection outputs generated by the first side beamdetectors 30B of the respective first and second photo-detectors 24A and24B are indicated by E14 and E15, and detection outputs generated by thesecond side beam detectors 31C of the respective first and secondphoto-detectors 24A and 24B are indicated by E16 and E17, a trackingerror signal TE indicating a degree of offset of a scanning light beamfrom the aimed recording track on the first information signal storagelayer 5A of the two-layer optical disc 5 from which the informationsignal is intended to be read out, can be obtained according to thefollowing equation (8):

    TE=(E14+E15)-(E16+E17)                                     (8)

The main beam 31A, the first side beam 31B and the second side beam 31Call reflected from the first information signal storage layer 5A or thesecond information signal storage layer 5B on which the light beam isfocused to read out the information signal stored thereon, are incidenton the main beam detector 30A, the first side beam detector 30B and thesecond beam detector 30C, respectively. In addition to these focusedlight beams, the main beam detector 30A, the first side beam detector30B and the second beam detector 30C also receive respectively the straylight 32A attributed to the main beam, and the stray lights 32B and 32Cattributed to the first and second side beams. These stray beams are allreflected from the second information signal storage layer 5B or thefirst information signal storage layer 5A on which the light beam is notfocused and from which no information signal is intended to be read out.

In this case, the stray beam 32A attributed to the main beam which isreflected from the information signal storage layer 5A or 5B from whichno information signal is intended to be read out, and radiated onto themain beam detector 30A, is not radiated over the first and second sidebeam detectors 30B and 30C in an overlapped relation to the side beams.Similarly, the stray lights 32B and 32C attributed to the first andsecond side beams, are not radiated over the main beam detector 30A inan overlapped relation to the main beam.

Even if the stray lights 32B and 32C attributed to the first and secondside beams are radiated over the main beam detector 30A, in anoverlapped relation to the main beam, the main beam detector 30A is notadversely affected by the radiation of side beams because a lightintensity of each side beam is small enough as compared with that of thestray light 32A attributed to the main beam.

As described above, in the optical pick-up device 3 according to thethird embodiment of the present invention, the first side beam detector30B and the second side beam detector 30C are disposed outside of thespot diameter of the stray light 32A attributed to the main beam, sothat the stray light 32A attributed to the main beam is not radiatedover the first and second side detectors 30B and 30C in an overlappedrelation to the side beams. Accordingly, in the optical pick-up device 3according to the present embodiment, D.C. offset of the tracking errorsignal is also prevented from occurring, whereby the tracking controlfor the objective lens 11 can be accurately carried out. Thus, therecording track is accurately tracked by the light beam being radiated.

Further, the optical pick-up device 3 according the present embodimentis preventing from suffering from D.C. offset of the tracking errorsignal irrespective of whether an information signal to be reproduced isstored on the first information signal storage layer 5A or the secondinformation signal storage layer 5B. Accordingly, the tracking controlfor the objective lens 11 can be accurately carried out. Similarly, therecording track is accurately tracked by the light beam being radiated.

Furthermore, the optical pick-up device 3 according to the presentinvention is provided with the polarizing beam splitter 23, because adifferential concentric circles method is employed as a system fordetection of the focusing error. Accordingly, in the optical pick-updevice 3 according to the present embodiment, in case that themulti-layer optical disc used is of a magneto-optical type, thepolarizing beam splitter 23 also functions as an apparatus for analyzingthe information signal of the multi-layer magneto-optical disc. Thus,the polarizing beam splitter 23 serves for detecting both the focusingerror signal and the information signal of the multi-layermagneto-optical disc. Accordingly, simplification in an entire structureof the apparatus is accomplished, so that reduction in size of an entirebody of the apparatus can be realized.

In the optical pick-up device 3 according to the third embodiment of thepresent invention, the light beam emitted from the semiconductor laser 6is directed to the diffraction grating 18 where the light beam isdivided three separate beams including the main beam 31A, the first sidebeam 31B and the second side beam 31C. The main beam 31A, the first sidebeam 31B and the second side beam 31C are radiated to the main beamdetector 30A, the first side beam detector 30B and the second side beamdetector 30C, respectively. The main beam detector 30A, the first sidebeam detector 30B and the second side beam detector 30C are disposedspaced apart from each other by the constant distance A₂ correspondingto that between the main beam 31A, the first side beam 31B and thesecond side beam 31C. In the foregoing, there is described an opticalpick-up device 4 according to a fourth embodiment of the presentinvention, which has such a simplified construction that the stray lightattributed to the main beam and radiated over the first and second sidebeam detectors does not adversely affect the function thereof.

Incidentally, some elements or members of the optical pick-up device 4according to the fourth embodiment of the present invention areidentical to those of the optical pick-up device 3 according to thethird embodiment. Therefore, like reference numerals denote like partsand the detailed explanations thereof are omitted.

A photo-detector 37 of the optical pick-up device includes, as shown inFIG. 11, a main beam detector 38A and first and second side beamdetectors 38B and 38C disposed adjacent to the main beam detector 38A onthe opposite sides thereof such that the main beam detector 38A isinterposed therebetween.

As shown in FIG. 11, the main beam detector 38A has a light-receivingsurface which is divided into three strip-like light-receiving regionsdisposed in parallel with each other and composed of first, second andthird light-receiving regions 50A, 50B and 50C. The first and secondside beam detectors 38B and 38C used in this embodiment each have asingle non-separated light-receiving region.

The light-receiving region of the first side beam detector 38B has acut-out portion on a side facing the main beam detector 38A where thestray light 32A attributed to the main beam and directed to the mainbeam detector 38A might be radiated over the side beam detector in anoverlapped relation to the side beams. Similarly, the light-receivingregion of the second side beam detector 38C has a cut-out portion on aside facing the main beam detector 38A where the stray light 32Aattributed to the main beam and directed to the main beam detector 38Amight be radiated over the side beam detectors in an overlapped relationto the side beams.

In the afore-mentioned arrangement of the optical pick-up deviceaccording to the fourth embodiment of the present invention, since apart of each light-receiving surface of the first and second side beamdetectors 38B and 38C which faces the main beam detector 38A, is cut outto avoid the overlapped radiation of the stray light 32A attributed tothe main beam, each side beam detector is effectively prevented frombeing adversely affected by the stray light 32A attributed to the mainbeam.

Meanwhile, in the afore-mentioned optical pick-up devices according tothe preferred embodiments of the present invention, the focusing errorsignal is detected by using an astigmatism method or a differentialconcentric circles method. However, the detection of the focusing errorsignal can be also performed by using Foucault's method, a criticalangle method, a knife-edge method, or the like.

Industrial Applicability

As described above, the optical pick-up device according to the presentinvention includes a light source for emitting a light beam to beradiated toward a multi-layer optical disc having a plurality ofinformation signal storage layers which are disposed in an overlappedmanner and on each of which a desired information signal is recorded, anoptical system for dividing the light beam emitted from the light sourceinto three separate beams composed of a main beam and first and secondside beams and radiating these beams onto the multi-layer optical disc,a main beam-receiving member for receiving the main beam reflected fromthe multi-layer optical disc, a first side beam-receiving member forreceiving the first side beam reflected from the multi-layer opticaldisc, a second side beam-receiving member for receiving the second sidebeam reflected from the multi-layer optical disc, and a means foroutputting a detection signal indicative of a quantity of light inresponse to the light quantity received by the first and second sidebeam detectors. Further, the first side beam detector and the secondside beam detector are disposed spaced by a given distance apart fromthe main beam detector at positions where no interference with the mainbeam reflected from an unfocused information signal storage layer of themulti-layer optical disc, is caused. As a result, the light beamreflected from the unfocused information signal storage layer of themulti-layer optical disc, does not have any influence on the function ofthe device. Accordingly, a desired information signal can be accuratelyreproduced from the intended information signal storage layer on whichthe information signal is recorded.

What is claimed is:
 1. An optical pick-up device for a multi-layeroptical disc comprising:a light source for emitting a light beam to beradiated toward said multi-layer optical disc having a plurality ofinformation signal storage layers which are arranged in an overlappedmanner and on each of which a desired information signal is recorded; anoptical means for dividing the light beam emitted from the light sourceinto three separate beams composed of a main beam and first and secondside beams and radiating these beams onto the multi-layer optical disc;a main beam-receiving means for receiving the main beam reflected fromthe multi-layer optical disc; a first side beam-receiving means forreceiving the first side beam reflected from the multi-layer opticaldisc; a second side beam-receiving means for receiving the second sidebeam reflected from the multi-layer optical disc; and a means foroutputting a detection signal indicative of a quantity of light inresponse to the light quantity received by each of the mainbeam-receiving means, the first side beam-receiving means and the secondside beam-receiving means, said first side beam-receiving means and saidsecond side beam-receiving means being spaced a distance apart from saidmain beam-receiving means so that no interference with said first andsecond side beam-receiving means results from the main beam reflectedfrom an unfocused information signal storage layer of the multi-layeroptical disc which is incident on said main beam-receiving means.
 2. Anoptical pick-up device for a multi-layer optical disc as claimed inclaim 1, wherein said optical means for dividing the light comprises adiffraction grating, said diffraction grating dividing the light beamemitted from the light source into said three separate beams which arespaced apart from each other by such a distance that the main beamreflected from an unfocused information signal storage layer does notinterfere with said first side beam-receiving means and said second sidebeam-receiving means.
 3. An optical pick-up device for a multi-layeroptical disc as claimed in claim 1, wherein detection of a focusingerror is conducted based on a detection output from said mainbeam-receiving means.
 4. An optical pick-up device for a multi-layeroptical disc as claimed in claim 3, further comprising an opticalelement for generating an astigmatism.
 5. An optical pick-up device fora multi-layer optical disc as claimed in claim 1, wherein detection of atracking error is conducted based on a detection output from said firstside beam-receiving means and said second side beam-receiving means. 6.An optical pick-up device for a multi-layer optical disc as claimed inclaim 1, wherein said first side beam-receiving means and said secondside beam-receiving means each have a cut-out portion on a side adjacentto said main beam-receiving means to prevent any of said main beamreflected from an unfocused information signal storage layer from beingincident on said first and second side beam-receiving means.
 7. Anoptical pick-up device for a multi-layer optical disc as claimed inclaim 5, wherein said tracking error is equal to a difference between aquantity of light received by said first side beam-receiving means andsaid second side beam-receiving means.
 8. An optical pick-up device fora multi-layer optical disc as claimed in claim 1, further comprising anoptical element interposed between said light source and said mainbeam-receiving means, wherein said optical element causes an astigmatismbetween light of said main beam reflected from a focused informationsignal storage layer and from said unfocused information signal storagelayer.
 9. An optical pick-up device for a multi-layer optical disccomprising:a light source for emitting a light beam to be radiatedtoward said multi-layer optical disc having a plurality of informationsignal storage layers which are arranged in an overlapped manner and oneach of which a desired information signal is recorded; an optical meansincluding a diffraction grating for dividing the light beam emitted fromthe light source into three separate beams composed of a main beam andfirst and second side beams, and an objective lens for radiating thethree separate beams divided by the diffraction grating onto themulti-layer optical disc in such a manner that the respective beams arefocused on an information signal storage layer of the multi-layeroptical disc; a main beam-receiving means for receiving the main beamreflected from the multi-layer optical disc; a first side beam-receivingmeans for receiving the first side beam reflected from the multi-layeroptical disc; a second side beam-receiving means for receiving thesecond side beam reflected from the multi-layer optical disc; and ameans for outputting a detection signal indicative of a quantity oflight in response to the light quantity received by each of the mainbeam-receiving means, the first side beam-receiving means and the secondside beam-receiving means, said first side beam-receiving means and saidsecond side beam-receiving means being spaced a distance apart from saidmain beam-receiving means so that no interference with said first andsecond side beam-receiving means results from the main beam reflectedfrom an unfocused information signal storage layer of the multi-layeroptical disc which is incident on said main beam-receiving means.
 10. Anoptical pick-up device for a multi-layer optical disc as claimed inclaim 9, wherein said diffraction grating divides the light beam emittedfrom the light source into said three separate beams which are spacedapart from each other by such a distance that the main beam reflectedfrom an unfocused information signal storage layer causes nointerference with said first side beam-receiving means and said secondside beam-receiving means.
 11. An optical pick-up device for amulti-layer optical disc as claimed in claim 9, wherein detection of afocusing error is conducted based on a detection output from said mainbeam-receiving means.
 12. An optical pick-up device for a multi-layeroptical disc as claimed in claim 9, wherein detection of a trackingerror is conducted based on a detection output from said first sidebeam-receiving means and said second side beam-receiving means.
 13. Anoptical pick-up device for a multi-layer optical disc as claimed inclaim 11, further comprising an optical element for generating anastigmatism.
 14. An optical pick-up device for a multi-layer opticaldisc as claimed in claim 9, wherein said first side beam-receiving meansand said second side beam-receiving means each have a cut-out portion ona side adjacent to said main beam-receiving means to prevent any of saidmain beam reflected from an unfocused information signal storage layerfrom being incident on said first and second side beam-receiving means.15. An optical pick-up device for a multi-layer optical disccomprising:a light source for emitting a light beam to be radiatedtoward said multi-layer optical disc having a plurality of informationsignal storage layers which are arranged in an overlapped manner and oneach of which a desired information signal is recorded; an optical meansincluding a diffraction grating for dividing the light beam emitted fromthe light source into three separate beams composed of a main beam andfirst and second side beams, an objective lens for radiating the threeseparate beams divided by the diffraction grating onto the multi-layeroptical disc in such a manner that the respective beams are focused onthe information signal storage layer of the multi-layer optical disc,and a polarizing beam splitter on which the light beam reflected fromthe multi-layer optical disc is incident; first and secondphoto-detectors each having a main beam-receiving means for receivingthe main beam reflected from the multi-layer optical disc andtransmitted through or reflected on said polarizing beam splitter, afirst side beam-receiving means for receiving the first side beamreflected from the multi-layer optical disc and transmitted through orreflected on said polarizing beam splitter, and a second sidebeam-receiving means for receiving the second side beam reflected fromthe multi-layer optical disc and transmitted through or reflected onsaid polarizing beam splitter; and a means for outputting a detectionsignal indicative of a quantity of light in response to the lightquantity received by each of the main beam-receiving means, the firstside beam-receiving means and the second side beam-receiving meansprovided on each of said first and second photo-detectors, said firstside beam-receiving means and said second side beam-receiving means,provided on each of said first and second photo-detectors, being spaceda distance apart from said main beam-receiving means so that nointerference with said first and second side beam-receiving meansresults from the main beam reflected from an unfocused informationsignal storage layer of the multi-layer optical disc which is incidenton said main beam-receiving means.
 16. An optical pick-up device for amulti-layer optical disc as claimed in claim 15, wherein saiddiffraction grating divides the light beam emitted from the light sourceinto said three separate beams which are spaced apart from each other bysuch a distance that the main beam reflected from an unfocusedinformation signal storage layer causes no interference with said firstside beam-receiving means and said second side beam-receiving means. 17.An optical pick-up device for a multi-layer optical disc as claimed inclaim 15, wherein detection of a focusing error is conducted based on adetection output from said main beam-receiving means provided on each ofsaid first and second photo-detectors.
 18. An optical pick-up device fora multi-layer optical disc as claimed in claim 15, wherein detection ofa tracking error is conducted based on a detection output from saidfirst side beam-receiving means and said second side beam-receivingmeans provided on each of said first and second photo-detectors.
 19. Anoptical pick-up device for a multi-layer optical disc as claimed inclaim 15, wherein said main beam-receiving means provided on each ofsaid first and second photo-detectors is divided into threelight-receiving members.
 20. An optical pick-up device for a multi-layeroptical disc as claimed in claim 18, wherein said tracking error isequal to (E14+E15)-(E16+E17), where:E14 is a quantity of light receivedby said first side beam-receiving means of said first photo-detector;E15 is a quantity of light received by said first side beam-receivingmeans of said second photo-detector; E16 is a quantity of light receivedby said second side beam-receiving means of said first photo-detector;and E17 is a quantity of light received by said second sidebeam-receiving means of said second photo-detector.